1. Field of the Invention
The present invention relates to manufacturing methods of a thin film transistor and a display device including the thin film transistor.
2. Description of the Related Art
In recent years, thin film transistors that are formed using a semiconductor thin film having a thickness of several nanometers to several hundreds of nanometers over a substrate having an insulating surface such as a glass substrate have been attracting attentions. Thin film transistors are widely used for electronic devices such as ICs (integrated circuits) and electro-optical devices. In particular, thin film transistors are urgently developed as switching elements of display devices typified by liquid crystal display devices, electro luminescence (EL) display devices, and the like. In active matrix liquid crystal display devices, specifically, voltage is applied between a pixel electrode connected to a selected switching element and a counter electrode corresponding to the pixel electrode, and thus, a liquid crystal layer disposed between the pixel electrode and the counter electrode is aligned differently to be modulated optically. The optical modulation can be recognized as a display pattern by an observer. The active matrix liquid crystal display devices mentioned here form a display pattern on a screen by driving pixels that are arranged in a matrix form, with use of switching elements. The active matrix EL display devices mentioned here form a display pattern on a screen by driving pixels that are arranged in a matrix form, with use of switching elements.
The application of the above-described active matrix liquid crystal display devices and EL display devices is expanding, and demands for larger screen size, higher definition, and higher aperture ratio are increasing. In addition, it is demanded that the active matrix liquid crystal display devices and EL display devices have high reliability and that a production method of the active matrix liquid crystal display devices and EL display devices offer high yield and reduce production cost. In order to increase productivity and reduce production cost, the process can be simplified.
In the active matrix liquid crystal display devices and EL display devices, thin film transistors are mainly used as switching elements. In a manufacturing process of the thin film transistors, reduction in the number of photomasks used in photolithography is important for simplification of the process. For example, addition of one photomask needs the following steps: resist application, prebaking, light exposure, development, postbaking, and the like, and other steps before and after the aforementioned steps, such as film formation, etching, resist removal, cleaning, drying, and the like. Since the number of steps is thus significantly increased only by adding one photomask in the manufacturing process, many techniques for reducing the number of photomasks in the manufacturing process have been developed.
The thin film transistors are broadly classified into a top-gate type in which a gate electrode is formed over a channel formation region and a bottom-gate type in which a gate electrode is formed under a channel formation region. It is known that the number of photomasks used in a manufacturing process of bottom-gate thin film transistors is less than the number of photomasks used in a manufacturing process of top-gate thin film transistors. In general, three photomasks are used for manufacturing a bottom-gate thin film transistor.
Many conventional techniques for reducing the number of photomasks use a complicated technique such as backside light exposure, resist reflow, or a lift-off method, which requires a special apparatus. Using such complicated techniques may cause various problems, which leads to reduction in yield. Further, in many cases, reduction in the number of photomasks is at the expense of electric characteristics of a thin film transistor.
A widely known typical means for reducing the number of photomasks used for manufacturing a thin film transistor is a technique using a multi-tone mask (called a half-tone mask or a gray-tone mask). For example, Patent Document 1 discloses a technique for reducing the number of manufacturing steps by using a multi-tone mask.